The present invention relates to a method for the preparation of orientation layers for liquid crystal displays wherein a solution of an organic prepolymer is applied to a transparent substrate and subsequently annealed and subjected to an orientation treatment, and further relates to liquid crystal displays having an orientation layer made by this method.
In liquid crystal displays (LCD), uniform orientation of the liquid crystals is required. Accordingly, so-called "orientation layers" are applied to the electrodes. The preparation of the orientation layers can be accomplished, for example, by an oblique vapor deposition of SiO.sub.x. This process, however, is elaborate and relatively expensive. It has, therefore, been attempted to employ orientation layers made from organic materials.
A good seal is required in liquid crystal displays to ensure operating reliability and, thus, particularly in high quality displays, sealing with glass solder (a low-melting glass) is employed instead of cementing. As a consequence, the orientation layers used in glass-solder liquid crystal displays must be able to withstand temperatures up to 400.degree. C. and more without change in their morphological structure.
Organic orientation layers for liquid crystal indicators or displays consist, for example, of polyvinyl alcohol. However, such layers have only very limited dimensional heat stability. For increased thermal requirements, therefore, orientation layers made of polyimides are used. The orientation layers of polyimide are prepared in a manner such that a polyamido carboxylic acid (a prepolymer of polyimides) is dissolved in a solvent and is applied as a film to the electrode layer or electrode base plate. After the application, the film coat is heated to elevated temperatures, i.e., annealed, whereby the polyimide is formed from the polyamido carboxylic acid. This is then followed by the orientation treatment of the polyimide resin film (see in this connection U.S. Pat. No. 4,068,923).
Polyimide can be used only in small layer thicknesses, however, due to its intrinsic coloring. Since orientation layers with a polyimide base also exhibit insufficient dimensional heat stability at temperatures above 420.degree. C., they are poorly suited for high-quality liquid crystal displays fused with glass solder because the structure of the polyimide layers and, thus, the orientation of the liquid crystals in the display, is lost when the glass solder is fused.